It has been found that, when the brakes of a transit vehicle of a railway train consist are applied, the braking force must be properly controlled in order to safely and efficiently slow down and/or stop the vehicle or train at a station or the like. For a given running surface condition, the force between the wheel tread and the running surface or track increases during the initial stages of slippage. As slippage increases the slip value moves toward the a critical wheel slip value. When the value of the wheel slip increases beyond such critical slip value, the force between the wheel tread and the running surface decreases. It will be appreciated that stable and effective braking occurs when the slip value is either equal to or less than the critical slip value. Thus, when a slip value becomes greater than the critical slip value, the braking becomes unstable and can result in a sudden wheel lockup which can not only cause excessive wheel and running surface wear and wheel flattening, but will increase the actual stopping distance. Accordingly, in the braking operation, it is advantageous to detect an incipient wheel lockup by continuously monitoring the wheel slip value between the wheel tread and the running surface and in order to reduce and/or control the braking force being applied to the extent necessary to achieve the maximum possible braking effort without causing wheel lockup.
A prior art type wheel-slip control system is shown and disclosed in U.S. Pat. No. 4,491,920, issued on Jan. 1, 1985, entitled "Rate Polarity Shift Wheel-Slip Control System", which is assigned to the assignee of the present invention, and the teachings therein are incorporated herein by reference thereto in the present application. Briefly, the wheel-slip control in that patented system is for a multiple-truck vehicle and includes a speed sensor for generating signals representative of the velocity of each of the wheel axle units. A differentiator is connected to each of the speed sensors for differentiating the velocity signals to obtain a rate signal. A rate determining circuit determines the most negative-going rate signal of each of the wheel axle units of each truck. A plurality of deceleration threshold and rate direction detectors and data processing logic initiate a brake force reduction action on the truck experiencing a wheel slip, and a positive logic "OR" gate senses a polarity shift in the most negative rate signal to cause the data processing logic to reapply a braking action.
A more recent prior art wheel-slip control system is shown and disclosed in our U.S. Pat. No. 4,941,099, issued on Jul. 10, 1990, entitled "Electronic Adhesion Adaptive Wheel Slide Protection Arrangement Function", which is also assigned to the assignee of this invention, and is also incorporated herein by reference thereto. Pursuant to that patented system, a distinct and separate speed varying rate detection circuit and an energy storage slip detection circuit are provided, with the speed varying rate detection circuit being a part of a primary wheel slip detection system, while the energy storage slip detection circuit is a part of a synchronous wheel slip detection system. Each detection circuit is interfaced with its own separate pattern recognition logic, which are then tied together through a relatively complex brake force modulation scheme.
Accordingly, the above patented system uses two separate detection systems, with each detection system requiring a separate slip control logic, which then must be tied together with a rather complex force modulation scheme. In addition, the above described patented system uses a single energy storage level set point which limits the effectiveness of the system to a particular speed range, and is, therefore, normally used only for fast breakaway slippage, and is sometimes prone to over sensitivity.